Capacitors are storage devices that store electrical energy on an electrode surface. Electrochemical cells create an electrical charge at electrodes by chemical reaction. The ability to store or create electrical charge is a function of electrode surface area in both applications. Ultracapacitors, sometimes referred to as double layer capacitors, are a third type of storage device. An ultracapacitor creates and stores energy by microscopic charge separation at an electrical chemical interface between electrode and electrolyte.
Ultracapacitors are able to store more energy per weight than traditional capacitors and they typically deliver the energy at a higher power rating than many rechargeable batteries. Ultracapacitors comprise two porous electrodes that are isolated from electrical contact by a porous separator. The separator and the electrodes are impregnated with an electrolytic solution, which allows ionic current to flow between the electrodes while preventing electronic current from discharging the cell. Each electrode is in intimate contact with a current collector. One purpose of the current collector is to reduce ohmic loss. If the current collectors are nonporous, they can also be used as part of the capacitor case and seal.
When electric potential is applied to an ultracapacitor cell, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Upon reaching the electrode surface, the ionic charge accumulates to create a layer at the solid liquid interface region. This is accomplished by absorption of the charge species themselves and by realignment of dipoles of the solvent molecule. The absorbed charge is held in this region by opposite charges in the solid electrode to generate an electrode potential. This potential increases in a generally linear fashion with the quantity of charge species or ions stored on the electrode surfaces. During discharge, the electrode potential or voltage that exists across the ultracapacitor electrodes causes ionic current to flow as anions are discharged from the surface of the positive electrode and cations are discharged from the surface of the negative electrode while an electronic current flows through an external circuit between electrode current collectors.
In summary, the ultracapacitor stores energy by separation of positive and negative charges at the interface between electrode and electrolyte. An electrical double layer at this location consists of sorbed ions on the electrode as well as solvated ions. Proximity between the electrodes and solvated ions is limited by a separation sheath to create positive and negative charges separated by a distance which produces a true capacitance in the electrical sense.
During use, an ultracapacitor cell is discharged by connecting the electrical connectors to an electrical device such as a portable radio, an electric motor, light emitting diode or other electrical device. The ultracapacitor is not a primary cell but can be recharged. The process of charging and discharging may be repeated over and over. For example, after discharging an ultracapacitor by powering an electrical device, the ultracapacitor can be recharged by supplying potential to the connectors.
The physical processes involved in energy storage in an ultracapacitor are distinctly different from the electrochemical oxidation/reduction processes responsible for charge storage in batteries. Further unlike parallel plate capacitors, ultracapacitors store charge at an atomic level between electrode and electrolyte. The double layer charge storage mechanism of an ultracapacitor is highly efficient and can produce high specific capacitance, up to several hundred Farads per cubic centimeter.
Undesirable gas is generated in an ultracapacitor by a variety of mechanisms. First, water that is present within a sealed ultracapacitor component can be electrolyzed as the cell voltage rises above about 1.2 volts. Electrolyzing water produces oxygen gas and hydrogen gas. Additionally, gas can be generated from desorption from the carbon electrodes or from decomposition of electrolyte impurities. Gas in the ultracapacitor is undesirable for a number of reasons. A major reason is that the gas resides within pores of the carbon electrode or separator to increase electrical resistance.
The present invention relates to a method of preparing a nonaqueous ultracapacitor cell and particularly to a method for printing to provide an electrode that lessens substantially the adverse effect of gas produced within an ultracapacitor.
The method of producing the electrode according to the present invention is advantageous in another respect. The method is a screen printing method. Heretofore, screen printing methods were restricted to electrodes of about 6 inches or less on a side because larger electrodes would cause the printing screen to deform from the weight of the screen printing wiper blade. Screen deformation results in an unequal application of electrode material. The present invention overcomes this deficiency in the art as well as the problem of gas accumulation.